The present invention is directed to a method and device for determining the focal length of long focal length electron optical lenses especially electron optical lenses utilized in a microprojector.
In the manufacture of highly integrated switching circuits, a microprojector is utilized so that an image of a large area transmission mask can be projected on a photosensitive surface layer on a wafer, for example a monocrystalline silicon wafer as a desired reduced image and after processing to form a desired structure, additional images are projected on additional photosensitive layer to produce subsequent structural features. With progressive miniaturization, the demands on the imaging system increase. If the structures with a position accuracy of 0.1 .mu.m are to be produced on an image field of 5 mm, the allowable error in alignment relative to the image field diameter amounts for approximately 2.times.10.sup.-5. In order to be able to obtain this accuracy even in the case of repeated exposures and for a setting of the microprojector, the focal length of the lenses used for the paths of rays and especially for an enlargement must be known very precisely.
In the case of a conventional electron optical instrument, for example an electron microscope, the focal length can be determined through magnification measurements with a known test object. Due to the high possible magnification values and the lower accuracy requirement which is present in an electron microscope but not in a microprojector, this method is sufficient for conventional electron optical instruments. In the case of microprojectors, on the other hand, the image of a transmission mask is reduced only approximately by a factor of 1:10 onto the layer of the wafer. The known method for focal length measurement therefore does not provide the required accuracy.